The present invention relates to an engine and, more particularly, to a lean burn engine system in which swirls are generated in the combustion chamber corresponding to the operating state of the engine to improve the combustion even in a lean air-fuel ratio.
The prior art of improving combustion capability by generating swirls in the combustion chamber of an engine is known. Further, there is also known a technology to improve fuel consumption rate in which lean air-fuel ratio operation of an engine is performed during a low load operating period in order to decrease pumping loss of the engine.
By combining the above technologies, during a low load operating period an engine is operated under condition of lean mixture and generating swirls to improve the combustion capability for decreasing fuel consumption rate, which is well known as so-called lean burn engine technology.
In the technology of this type, the swirl is preferably adjusted in an optimum strength corresponding to the operating condition of the engine. More particularly, during a low load operating period the swirl is strengthened to improve the combustion capability, and during a high load operating period the swirl is weakened to suck a great amount of air to increase the output power. in order to realize the above, a technology is described in, for example, Japanese Patent Application Laid-Open No.61-58921 (1986) where an engine of two intake valve type is constructed such that one of the intake valves composes a straight port having an intake control valve, and the other composes a helical port having a bypass passage opened by the intake control valve. With this construction, during a low load operating period, the intake control valve is held close to suck air through only the passage in the side of the helical port to generate a strong swirl in the combustion chamber. On the other hand, during a high load operating period, the intake control valve is held open to suck air through the helical port and the straight port and further through a bypass passage communicating from the straight port side to the helical port side to increase the output power by increasing the amount of intake air.
However, in such an engine constructed as above, when the engine is operated under various conditions, there arise problems as follows.
Firstly, there is essentially only one mechanism to generate an optimum swirl, and consequently it is not possible to set operating conditions of the engine in which optimum swirl is generated except only one condition. Therefore, for example, when the engine is set so as to generate an optimum swirl during a low load operating period in a state of holding the intake control valve closed, during an intermediate load operating period of the engine the sufficient intake air flow rate cannot be supplied through only the helical port and consequently there arises a problem in that the operating range of the engine capable of operating with lean air-fuel ratio is narrowed. In the contrary, when the engine is set so as to generate an optimum swirl during an intermediate load operating period, during a low load operating period of the engine there arises a problem in that the swirl is weakened due to decrease in the speed of intake air flow.
During a high load operating period of the engine, there arises a problem in that the output power is decreased due to decrease in the intake air flow rate comparing to the case of a conventional engine which has two straight ports although a large amount of air flow is sucked through the helical port and the swirl is weakened by the bypass passage.
Further, in the engine constructed as described above, one large swirl is generated in the horizontal or in a tilting direction inside the-combustion chamber. However, there is a problem in that such a large swirl has a small effect to improve the combustion because the rotating energy of large swirl has relatively a small effect on mixing air and fuel. Especially in a spark ignition engine of fuel injecting type, in a case of having two intake valves as described above, the mist from a fuel injection valve is generally formed in bi-directional mist flows directing from the intake pipe to the intake valves. However, the mist is attached to the wall of the intake pipe passage or the wall of the combustion chamber. During an intermediate load operating period of the engine, the fuel is blown aside in the combustion chamber by the large swirl in the horizontal or in a tilting direction described above, which causes a problem in that the fuel near the wall is exhausted without burning to increase HC content in the exhaust gas, or the fuel consumption rate is increased. Furthermore, there is a problem in that only a part of the combustion chamber becomes at a high temperature during combustion period to increase NOx content in the exhaust gas.
The items of the problems to be solved by the present invention are as follows.
Firstly, to provide means for generating a swirl having a proper strength in a wide operating region of an engine.
Secondarily, to provide means which has a good suction characteristic to suppress decrease in the output power during a high load operating period of the engine, and is capable of generating a swirl having a proper strength during an intermediate and low load operating period.
Thirdly, to provide such a swirl and swirl generating means that the swirl is capable of effectively mixing fuel and air, the mist of the fuel does not attach to the wall of intake pipe or the wall of combustion chamber, the ignitability is good, the combustion efficiency is high, and the exhaust gases such as HC, NOx and the like are suppressed to be generated.
In order to solve the above problems, the present invention can provide the following means.
Initially, a plurality of sub-air-intake passages are provided separately from the main passage of air-intake pipe The total sum of the cross-sectional areas the sub-air-intake pipes is made smaller than the cross-sectional area of the main air-intake passage. The outlet of the sub-air-intake passage opens to a position near an intake valve inside the main air-intake passage, and is directed to the gap portion between the intake valve and a corresponding intake valve sheet such that air enters from the outer side opposite to the facing side of the two intake valve sheets. The fuel mist enters from the inner side of the two intake valve sheets facing to each other to prevent interference with the intake air when swirls are generated. The respective sub-air-intake passages are constructed such that air enters toward at least two positions inside the combustion chamber. The pipe diameter and the pipe length of the sub-air-intake passage is preferably determined in such a relation as to effectively utilize the inertia effect of the intake air.
Further, means for closing the main passage and means for closing at least one of the sub-air-intake passages depending on the operating state of the engine are provided.
During a low load operating period of the engine, the main passage and a part of the sub-air-intake passages are kept close, and, thereby, swirls are generated inside the combustion chamber by the air flowing through the remaining sub-air-intake passages. During an intermediate load operating period, the main passage is kept closed and number of sub-air-intake passages are increased, and, thereby, the need of increasing intake air flow rate is coped with and plural swirls having different center axes are generated in the combustion chamber. When the engine enters into a high load operation, an intake control valve in the main passage is opened and a large amount of intake air is introduced to keep the torque of the engine.
The ignition timing of the mixed gas is delayed comparing to the case of a conventional engine without swirl if the combustion speed is increased by the generated swirls. And In a state where the swirls are not generated such as at a high load operating state, the ignition timing is set to the same condition as in a conventional engine.
Therein, fuel is injected in advance of the ignition timing by the time interval during which the fuel mist is mixed with the generated swirls and the mixed portion reaches near a spark plug to be ignited. The device is constructed such that the direction of fuel injection is directed toward swirls when the swirls are generated.
By constructing the device as described above, the present invention has the following effects.
Firstly, since the cross-sectional area of the air-intake passage can be changed in multi-step by varying the number of sub-air-intake passages, the swirls generated inside the combustion chamber can be set to a proper strength within the wider operating region of the engine comparing to a conventional engine. Thereby, the total efficiency of combustion in various operating condition of the internal combustion engine can be improved.
Secondarily, the amount of intake air flow rate sucked through the sub-air-intake passages is increased due to the inertia effect of intake air through the sub-air-intake passages. Thereby, the region for operating the engine with generating swirls can be expanded.
Thirdly, since there is no need to provide swirl generating means such as a helical port in the main air passage, the air-intake flow resistance of the main passage is small and the larger amount of air can be sucked during a high load operating period
Fourthly, a plurality of swirls can be generated inside the combustion chamber by using a plurality of sub-air-intake passages. Thereby, the disturbance in the combustion chamber can be increased with the same intake-air flow rate comparing to the case where there is only one swirl, and consequently the mixing of air and fuel is promoted to improve the efficiency of combustion.
Fifthly, since the injection timing of fuel, the ignition timing and the direction of fuel injection are adjusted such that plural swirls are generated around fuel mist and at the same time the fuel is not interfered with intake air, the fuel can be prevented from blowing aside near the wall of the combustion chamber. Thereby, harmful components such as HC, NOx and the like in the exhaust gas are decreased.